Mass measurements and 3D orbital geometry of PSR J1933–6211

Abstract

PSR J1933−6211 is a pulsar with a spin period of 3.5 ms in a 12.8 d nearly circular orbit with a white dwarf companion. Its high proper motion and low dispersion measure result in such significant interstellar scintillation that detections with a high signal-to-noise ratio have required long observing durations or fortuitous timing. In this work, we turn to the sensitive MeerKAT telescope, and combined with historic Parkes data, are able to leverage the kinematic and relativistic effects of PSR J1933−6211 to constrain its 3D orbital geometry and the component masses. We obtain a precise proper motion magnitude of 12.42 (3) mas yr−1 and a parallax of 1.0 (3) mas, and we also measure their effects as secular changes in the Keplerian parameters of the orbit: a variation in the orbital period of 7 (1)×10−13 s s−1 and a change in the projected semi-major axis of 1.60 (5)×10−14 s s−1. A self-consistent analysis of all kinematic and relativistic effects yields a distance to the pulsar of $ 1.6^{+0.2}_{-0.3} $ kpc, an orbital inclination, i = 55 (1) deg, and a longitude of the ascending node, $ \Omega = 255^{+8}_{-14} $ deg. The probability densities for Ω and i and their symmetric counterparts, 180 − i and 360 − Ω, are seen to depend on the chosen fiducial orbit used to measure the time of passage of periastron (T0). We investigate this unexpected dependence and rule out software-related causes using simulations. Nevertheless, we constrain the masses of the pulsar and its companion to be $ 1.4^{+0.3}_{-0.2}\,M_\odot $ and 0.43 (5) M⊙, respectively. These results strongly disfavour a helium-dominated composition for the white dwarf companion. The similarity in the spin, orbital parameters, and companion masses of PSRs J1933−6211 and J1614−2230 suggests that these systems underwent case A Roche-lobe overflow, an extended evolutionary process that occurs while the companion star is still on the main sequence. However, PSR J1933−6211 has not accreted significant matter: its mass is still at ∼1.4 M⊙. This highlights the low accretion efficiency of the spin-up process and suggests that observed neutron star masses are mostly a result of supernova physics, with minimum influence of subsequent binary evolution.